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WOW that is a question that I have wundered for a long time. Have three tanks with crawdads. Have fed them dried egg shells, always leave the molted shells in the tank. Have contemplated getting ground oyster shells from the feed store but don`t know if they are clean or not. Manny questions come to mind with crawdads. Hope that helps some.

I dont think adding calcium would hurt.
I have a crayfish and a snail in the same tank.. I feed them vegetables that contain calcium, and fruits. I also put tums in the tank - as in the chewable for stomach aches yes! Its very high in calcium and Ive watched it repair my snails shell. Was not expecting my cray to like the tums (it was for my snail) so I put half of one in the first time and the snail was eating it and my crayfish came along pushed the snail out of the way and took off with the tums haha ever since then I put a tums in weekly split it in too and make sure the cray has his first.

Yeah, I'm sure it can't be healthy to do it often..
But I can honestly say I've had no problems so far and I've been doing it for about a month in a half. My snails shell is fully repaired and its had plenty of new growth since then. It totally clouds my tank though and you need to do a water change after their done with it or the next day. Haven't noticed anything negative yet though.